The present invention relates to an arrangement for induction heating, especially for butt-welding of two primarily pipe- or bolt-shaped elements of metal which are arranged close to each other along substantially the same center axis, and which at their welding edges define a narrow gap or joint which is kept approximately constant during the heating phase.
In connection with butt-welding of pipe- or rod-shaped members of metal, there are previously suggested high frequency resistance heating as well as induction heating.
A special embodiment of resistance heating is disclosed in Norwegian patent application No. 850049, and in practice this has proved effective and appropriate in connection with the welding of pipe elements having a diameter up to 16". What is required is a high frequency generator having a power of approximately 300 kV and a frequency of about 20.000 Hz. Larger pipe diameters, for example in the range 42" and pipe thicknesses of 1-1 1/4" will however require a high frequency generator of approximately 800 kV. In connection with larger, continuous welding operations, for example when welding together pipe elements to long pipe lines, it will be required to have an extra high frequency generator serving as a stand-in in case the one generator should fail or must be repaired. Such high frequency generators are very expensive and will accordingly substantiate the main investment in a high frequency resistance welding system.
Further, from Norwegian patent application 833729 there is known a method for joining parts of metal by forge welding, wherein the parts prior to the forge welding is heated by means of an induction coil. Due to rational joint welding, it is of great importance that the heating is concentrated as much as possible in the area of the joint. If the heat is allowed to spread too far from the joint, the heating time will increase, the bending force of the parts will be reduced during he forge welding, the favorable tri-axial stress condition will be reduced, and the necessity of heat treatment after the completed welding will increase.